🚀 go-pugleaf

In article <38r8kh$5n0@delphinium.cig.mot.com> svoboda@rtsg.mot.com (David Svoboda) writes:

#NASA has become a very political body, of late, competing with other more
#publicly-valued federal government programs for shrinking funds.  They have
#a motive for making pronoucements which capture sensational public notice.

This has very little to do with the factual merits of the case, and
NASA is not the prime source of evidence for the ozone hole research.
See below.

#
#|Maybe YOU should study some chemistry. The great thing 'bout the CFC problem
#|is, that the second part of the intermediate reaction releases Cl which can
#|now react w/more O3. As far as the CFC issue goes, YOU ARE WRONG!
#|				-BIll Lane, w/some help from Dr. Dennis Anjo,
#|				phd. analytic chemistry

[half-smart garble omitted]

#Dave Svoboda (svoboda@ranger.rtsg.mot.com)  | I think God's got a sick

For many months now, (Prof.) Dave Parson of Colorado has been
putting up a FAQ in sci.environment detailing the evidence for
the key role artificial chlorine compounds in ozone depletion.
I reproduce a summary below. If you have specific rebuttals of
each point, by all means post them, with your documentation.
  Eric Mankin

________________________________________________________________

Subject: 7.) What is the evidence for the present theory?

The evidence is overwhelming - the results from a single 1987 
expedition (albeit a crucial one) fill two entire issues of the
Journal of Geophysical Research. What follows is a very sketchy
summary; for more information the reader is directed to [Solomon]
and to [Anderson et al.].

The theory described above (which is often called the
"PSC theory") was developed during the years 1985-87. At the same
time, others proposed completely different mechanisms, making no
use of chlorine chemistry. The two most prominent alternative
explanations were one that postulated large increases in nitrogen
oxides arising from enhanced solar activity, and one that
postulated an upwelling of ozone-poor air from the troposphere into
the cold stratospheric vortex.  Each hypothesis made definite
predictions, and a program of measurements was carried out to test
these. The solar activity hypothesis predicted enhanced NOx, whereas 
the measurements show unusually _low_ NOx ("denoxification), in 
accordance with the PSC hypothesis.  The "upwelling" hypothesis
predicted upward air motion in the lower stratosphere, which is
inconsistent with measurements of atmospheric tracers such as
N2O which show that the motion is primarily downwards.

Positive evidence for the PSC theory comes from ground-based and 
airborne observations of the various chlorine-containing compounds. 
These show that the reservoir species HCl and ClONO2 are extensively 
depleted in the antarctic winter and spring, while the concentration 
of the active, ozone-depleting species ClO is strongly enhanced. 
Measurements also show enormously enhanced concentrations of the 
molecule OClO. This is formed by a side-reaction in the BrO/ClO 
mechanism described above.

Further evidence comes from laboratory studies. The gas-phase
reactions have been reproduced in the laboratory, and shown to
proceed at the rates required in order for them to be important in
the polar stratosphere. [Molina et al. 1990] [Sander et al.] 
[Trolier et al.] [Anderson et al.]. The production of active
chlorine from reservoir chlorine on ice and sulfuric acid surfaces
has also been demonstrated in the laboratory [Tolbert et al.
1987,1988] [Molina et al. 1987]. (Recently evidence for these
reactions has been found in the arctic stratosphere as well: air
parcels that had passed through regions where the temperature 
was low enough to form PSC's were found to have anomalously 
low concentrations of HCl and anomalously high concentrations 
of ClO [AASE].)

The "smoking gun", however, is usually considered to be the
simultaneous in-situ measurements of a variety of trace gases from 
an ER-2 stratospheric aircraft (a converted U2 spy plane) in 
August-October 1987. [Tuck et al.] These measurements demonstrated a 
striking "anticorrelation" between local ozone concentrations and ClO 
concentrations. Upon entering the "hole", ClO concentrations 
suddenly jump by a factor of 20 or more, while ozone concentrations 
drop by more than 50%. Even the local fluctuations in the 
concentrations of the two species are anticorrelated.[Anderson et al.] 

In summary, the PSC theory explains the following observations:

1. The ozone hole occupies the region of the polar vortex where
   temperatures are below -80 C and where polar stratospheric clouds
   are abundant. 

2. The ozone hole is confined to the lower stratosphere.
 
3. The ozone hole appears when sunlight illuminates the vortex, and
   disappears soon after temperatures rise past -80 C, destroying PSC's.

4. The hole is associated with extremely low concentrations of NOx.

5. The hole is associated with very low concentrations of the chlorine
   "reservoirs", HCl and ClONO2, and very high concentrations of active
   chlorine compounds, ClO, and of byproducts such as OClO.

6. Inside the hole, the concentrations of ClO and ozone are precisely
    anticorrelated, high ClO being accompanied by low ozone.

7. Laboratory experiments demonstrate that chlorine reservoir compounds
   do react to give active chlorine on the surfaces of ice particles.

8. Airborne measurements in the arctic stratosphere show that air 
   which has passed through regions containing PSC's is low in 
   reservoir chlorine and high in active chlorine.

The antarctic ozone hole, once a complete mystery, is now 
one of the best understood aspects of the entire subject; it is 
much better understood than the small but steadily growing ozone
depletion at mid latitudes, for example.

-----------------------------
[how do the artificial chemicals get to the antarctic stratosphere?]

Subject: 2.1) What are the sources of chlorine in the troposphere?

Let us divide the chlorine-containing compounds found in the 
atmosphere into two groups, "organic chlorine" and "inorganic
chlorine". The most important inorganic chlorine compound in the 
troposphere is hydrogen chloride, HCl. Its principal source is 
acidification of salt spray - reaction of atmospheric sulfuric and 
nitric acids with chloride ions in aerosols.  At sea level, this 
leads to an HCl mixing ratio of 0.05 - 0.45 ppbv, depending strongly 
upon location (e.g. smaller values over land.) However, HCl dissolves 
very readily in water (giving hydrochloric acid), and condensation of 
water vapor  efficiently removes HCl from the _upper_ troposphere. 
Measurements show that the HCl mixing ratio is less than 0.1 ppbv at 
elevations above 7 km,  and less than 0.04 ppbv at 13.7 km. 
[Vierkorn-Rudolf et al.] [Harris et al.] 

There are many volatile organic compounds containing chlorine, but
most of them are quickly decomposed by the natural oxidants in the
troposphere, and the chlorine atoms that were in these compounds
eventually find their way into HCl or other soluble species and are 
rained out. The most important exceptions are:

 ChloroFluoroCarbons, of which the most important are
 CF2Cl2 (CFC-12), CFCl3 (CFC-11), and CF2ClCFCl2 (CFC-113);
 
 HydroChloroFluoroCarbons such as CHClF2 (HCFC-22); 

 Carbon Tetrachloride, CCl4; 

 Methyl Chloroform, CH3CCl3;

 and Methyl Chloride, CH3Cl (also called Chloromethane).  

Only the last has a large natural source; it is produced
biologically in the oceans and chemically from biomass burning. 
The CFC's and CCl4 are nearly inert in the troposphere, and have
lifetimes of 50-200+ years.  Their major "sink" is photolysis by UV
radiation. [Rowland 1989, 1991] The hydrogen-containing halocarbons
are more reactive, and are removed in the troposphere by reactions
with OH radicals. This process is slow, however, and they live long
enough (1-20 years) for a large fraction to reach the stratosphere.

As a result of this enormous difference in atmospheric lifetimes, 
there is more chlorine present in the lower atmosphere in 
halocarbons than in HCl, even though HCl is produced in much larger
quantities. Total tropospheric organic chlorine amounted to
~3.8 ppbv in 1989 [WMO 1991], and this mixing ratio is very nearly
independent of altitude throughout the troposphere. Methyl Chloride,
the only ozone-depleting chlorocarbon with a major natural source,
makes up 0.6 ppbv of this total. Compare this to the tropospheric HCl 
mixing ratios given above: < 0.5 ppbv at sea level, < 0.1 ppbv at 3 km, 
and < 0.04 ppbv at 10 km.  

-----------------------------

Subject: 2.2) In what molecules is _stratospheric_ chlorine found?

The halocarbons described above are all found in the stratosphere, 
and in the lower stratosphere they are the dominant form of chlorine.
At higher altitudes inorganic chlorine is abundant, most of it in 
the form of HCl or of _chlorine nitrate_, ClONO2.  These are called 
"chlorine reservoirs"; they do not themselves react with ozone, but 
they generate a small amount of chlorine-containing radicals - Cl,
ClO, ClO2, and related species, referred to collecively as the
"ClOx family" - which do. An increase in the concentration of 
chlorine reservoirs leads to an increase in the concentration of 
the ozone-destroying radicals.

-----------------------------

Subject: 2.3) What happens to organic chlorine in the stratosphere?

The organic chlorine compounds are dissociated by UV radiation
having wavelengths near 230 nm. Since these wavelengths are also
absorbed by oxygen and ozone, the organic compounds have to rise
high in the stratosphere in order for this photolysis to take
place. The initial (or, as chemists say, "nascent") products are 
a free chlorine atom and an organic radical, for example:

 CFCl3 + hv -> CFCl2 + Cl

The chlorine atom can react with methane to give HCl and a methyl
radical:

 Cl + CH4 -> HCl + CH3

 Alternatively, it can react with ozone and nitrogen oxides:

 Cl + O3 -> ClO + O2
 ClO + NO2 -> ClONO2

 (There are other pathways, but these are the most important.)
 
The other nascent product (CFCl2 in the above example) undergoes
a complicated sequence of reactions that also eventually leads to
HCl and ClONO2. Most of the inorganic chlorine in the stratosphere
therefore resides in one of these two "reservoirs". The immediate 
cause of the Antarctic ozone hole is an unusual sequence of 
reactions, catalyzed by polar stratospheric clouds, that "empty" 
these reservoirs and produce high concentrations of ozone-destroying 
ClOx radicals. [Wayne] [Rowland 1989, 1991]

-----------------------------

Subject: 2.4) How do we know that CFC's are photolyzed in the stratosphere?

The UV photolysis cross-sections for the halocarbons have been
measured in the laboratory; these tell us how rapidly they will
dissociate when exposed to light of a given wavelength and intensity.
We can combine this with the measured intensity of radiation in the
stratosphere and deduce the way in which the mixing ratio of a
given halocarbon should depend upon altitude. Since there is almost 
no 230 nm radiation in the troposphere or in the lowest parts of
the stratosphere, the mixing ratio should be independent of altitude
there. In the middle stratosphere the mixing ratio should drop off
quickly, at a rate which is determined by the photolysis cross-section.
Thus each halocarbon has a characteristic "signature" in its mixing
ratio profile, which can be calculated. Such calculations (first
carried out in the mid 1970's) agree well with the distributions
presented in the next section.

There is direct evidence as well. Photolysis removes a chlorine
atom, leaving behind a reactive halocarbon radical. The most likely
fate of this radical is reaction with oxygen, which starts a long
chain of reactions that eventually remove all the chlorine and
fluorine. Most of the intermediates are reactive free radicals, but
two of them, COF2 and COFCl, are fairly stable (they are analogs of
formaldehyde, H2CO) and live long enough to be detected - and have been.

-----------------------------

Subject: 2.5) How is chlorine removed from the stratosphere?

Since the stratosphere is very dry, water-soluble compounds are
not quickly washed out as they are in the troposphere. The
stratospheric lifetime of HCl is about 2 years; the principal
sink is transport back down to the troposphere.

-----------------------------

Subject: 2.6) How is chlorine distributed in the stratosphere?

Over the past 20 years an enormous effort has been devoted to
identifying sources and sinks of stratospheric chlorine.  The
concentrations of the major species have been measured as a 
function of altitude, by "in-situ" methods ( e.g. collection
filters carried on planes and balloons) and by spectroscopic
observations from aircraft, balloons, satellites, and the Space 
Shuttle. From all this work we now have a clear and consistent
picture of the processes that carry chlorine through the stratosphere.         

Let us begin by asking where inorganic chlorine is found. In the 
troposphere, the HCl mixing ratio decreased markedly with increasing
altitude. In the stratosphere, on the other hand,  it _increases_ with 
altitude, rapidly up to about 35 km, and then more slowly up to 55km
and beyond. This was noticed as early as 1976 [Farmer et al.] 
[Eyre and Roscoe] and has been confirmed repeatedly since. Chlorine
Nitrate (ClONO2), the other important inorganic chlorine compound in 
the stratosphere, also increases rapidly in the lower stratosphere, and
then falls off at higher altitudes.  These results strongly suggest
that HCl in the stratosphere is being _produced_ there, not drifting 
up from below.

Let us now look at the organic source gases.  Here, the data show
that the mixing ratios of the CFC's and CCl4 are _nearly independent 
of altitude_ in the troposphere, and _decrease rapidly with altitude_ 
in the stratosphere.  The mixing ratios of the more reactive 
hydrogenated compounds such as CH3CCl3 and CH3Cl drop off somewhat
in the troposphere, but also show a much more rapid decrease in
the stratosphere.  The turnover in organic chlorine correlates
nicely with the increase in inorganic chlorine, confirming the
hypothesis that CFC's are being photolyzed as they rise high enough
in the stratosphere to experience enough short-wavelength UV. At
the bottom of the stratosphere almost all of the chlorine is
organic, and at the top it is all inorganic. [Fabian et al. ] 
[Zander et al. 1987] [Zander et al. 1992] [Penkett et al.]

Finally, there are the stable reaction intermediates, COF2 and
COFCl. These have been found in the lower and middle stratosphere, 
exactly where one expects to find them if they are produced from 
organic source gases and eventually react to give inorganic chlorine.

For example, the following is extracted from Tables II and III of
[Zander et al. 1992]; they refer to 30 degrees N Latitude in 1985.
I have rearranged the tables and rounded some of the numbers, and
the arithmetic in the second table is my own.

   Organic Chlorine and Intermediates, Mixing ratios in ppbv

Alt.,  CH3Cl CCl4 CCl2F2 CCl3F CHClF2 CH3CCl3 C2F3Cl3  ||  COFCl
km
12.5  .580  .100  .310  .205  .066     .096    .021    ||  .004
15    .515  .085  .313  .190  .066     .084    .019    ||  .010
20    .350  .035  .300  .137  .061     .047    .013    ||  .035
25    .120   -    .175  .028  .053     .002    .004    ||  .077
30     -     -    .030   -    .042      -       -      ||  .029
40     -     -     -     -     -        -       -      ||   -


     Inorganic Chlorine and Totals, Mixing ratios in ppbv

Alt., HCl  ClONO2   ClO  HOCl   ||   Total Cl,  Total Cl,  Total Cl
                                ||   Inorganic   Organic
km                              ||
12.5   -     -       -     -    ||       -       2.63        2.63
15    .065   -       -     -    ||     0.065     2.50        2.56
20    .566  .212     -     -    ||     0.778     1.78        2.56
25   1.027  .849    .028  .032  ||     1.936     0.702       2.64
30   1.452 1.016    .107  .077  ||     2.652     0.131       2.78
40   2.213 0.010    .234  .142  ||     2.607       -         2.61

I have included the intermediate COFCl in the Total Organic column.
It should be noted that COFCl was not measured directly in this
experiment, although the related intermediate COF2 was.

This is just an excerpt. The original tables give results every 2.5km 
from 12.5 to 55km, together with a similar inventory for Fluorine.
Standard errors on total Cl were estimated to be 0.02-0.04 ppbv.    

Notice that the _total_ chlorine at any altitude is nearly constant
at ~2.5-2.8 ppbv. This is what we would expect if the sequence of
reactions that leads from organic sources to inorganic reservoirs 
was fast compared to vertical transport. Our picture, then, would be
of a swarm of organic chlorine molecules slowly spreading upwards 
through the stratosphere, being converted into inorganic reservoir
molecules as they climb. In fact this oversimplifies things -
photolysis pops off a single Cl atom which does reach its final
destination quickly, but the remaining Cl atoms are removed by a
sequence of slower reactions. Some of these reactions involve
compounds, such as NOx, which are not well-mixed; moreover, 
"horizontal" transport does not really take place along surfaces of 
constant altitude, so chemistry and atmospheric dynamics are in fact 
coupled together in a complicated way.  These are the sorts of issues 
that are addressed in atmospheric models.  Nevertheless, this simple
picture helps us to understand the qualitative trends, and 
quantitative models confirm the conclusions [McElroy and Salawich].

We conclude that most of the inorganic chlorine in the stratosphere
is _produced_ there, as the end product of photolysis of the organic
chlorine compounds.

-----------------------------

Subject: 2.7) What happens to the Fluorine from the CFC's?

Most of it ends up as Hydrogen Fluoride, HF. The total amount of HF
in the stratosphere increased by a factor of 3-4 between 1978 and
1989 [Zander et al., 1990] [Rinsland et al.]; the relative increase 
is larger for HF than for HCl (a factor of 2.2 over the same period)
because the natural source, and hence the baseline concentration,
is much smaller. For the same reason, the _ratio_ of HF to HCl has
increased, from 0.14 in 1977 to 0.23 in 1990. As discussed above, the
decomposition of CFC's in the stratosphere produces reaction
intermediates such as COF2 and COFCl which have been detected in the
stratosphere. COF2 in particular is relatively stable and makes a
significant contribution to the total fluorine; the total amount
of COF2 in the stratosphere increased by 60% between 1985 and 1992
[Zander et al. 1994] The total Fluorine budget,
as a function of altitude, adds up in much the same way as the 
chlorine budget. [Zander et al. 1992, 1994] [Luo et al.]

-----------------------------

Subject: 2.8) Summary of the Evidence

 a. Inorganic chlorine, primarily of natural origin, is efficiently
 removed from the troposphere; organic chlorine, primarily 
 anthropogenic, is not, and in the upper troposphere organic 
 chlorine dominates overwhelmingly.

 b. In the stratosphere, organic chlorine decreases with altitude, 
 since at higher altitudes there is more short-wave UV available to
 photolyze it. Inorganic chlorine _increases_ with altitude. 
 At the bottom of the stratosphere essentially all of the chlorine
 is organic, at the top it is all inorganic, and reaction 
 intermediates such as COF2 are found at intermediate altitudes.

 c. Both HCl and HF in the stratosphere have been increasing steadily,         
 in a correlated fashion, since they were first measured in the 1970's.
 Reaction intermediates such as COF2 are also increasing.

-----------------------------